1. Field of the Invention
The present invention relates to improvements of a flat yoke type DC machine.
2. Description of the Related Art
There is well known a DC motor using a flat yoke. Such a flat yoke type DC motor can utilize space more effectively than a DC motor using a round yoke of the same diameter. The use of the flat yoke type DC motor has increased recently for motorcar parts and other applications.
However, the flat yoke type DC motor has a clearance or air gap between the armature and the yoke, which is smaller than that of the round yoke. This reduces the magnetic reluctance against the cross magnetomotive force. Thus, the flat yoke type DC motor will be influenced by the armature to a greater degree than a round yoke DC motor of the same diameter.
FIG. 3 shows a round yoke type DC motor. FIG. 5 shows a flat yoke type DC motor having the same diameter as that of the round yoke type DC motor shown in FIG. 3. It is well-known in the art that each of the DC motors comprises an armature 10, permanent magnets 12 functioning as magnetic stator poles and a yoke 14.
As is apparent from these figures, the DC motor utilizing the flat yoke 14 shown in FIG. 5 can be reduced in size, but will have a much smaller clearance d between the armature 10 and the yoke 14, than the DC motor utilizing the round yoke 14 shown in FIG. 3. Therefore, the DC motor of FIG. 5 will have considerably lower magnetic reluctance against the cross magnetomotive force and will, therefore, be more highly influenced by the armature reaction. In other words, the magnetic flux generated by the armature coil becomes larger than the conventional level against the main magnetic flux generated by the magnetic stator poles 12. This tends to create a larger magnetic field in the rectified zone due to the cross magnetomotive force.
FIG. 4 shows a distribution of magnetic flux in the round yoke type DC motor shown in FIG. 3 FIG. 6 shows a distribution of magnetic flux in the flat yoke type DC motor shown in FIG. 5. In FIGS. 4 and 6, curve 100 represents a profile of main magnetic flux created by the magnetic stator poles 12 while curve 110 represents a profile of composite magnetic flux generated by combining the main magnetic flux with a cross magnetomotive force produced from the armature reaction. As is apparent from these figures, the distribution of the composite magnetic flux 110 in the rectified zone 200 becomes much higher in the flat yoke rather than the round yoke. As a result, a large magnetomotive force will be formed in the coil when it passes through the rectified zone 200. Each time the rectifying action has been terminated, sparks are produced between the brush and the commutator. This reduces the effective lives of the brush and commutator and also causes noise and motor vibrations.
In order to avoid the adverse affect from the armature reaction, it has been proposed to move the position of the brush to an electrically neutral position at which the composite magnetic field defined by the magnetic field generated from the armature coil and the magnetic field formed from the main magnetic flux becomes zero.
In such a motor its armature is rotated in the opposite, clockwise and counter-clockwise directions, the electriclly neutral point is changed depending on the direction of rotation in the armature. It is impossible to shift the brush position to the electrically neutral point at each time the direction of rotation is changed from one to another.
Even in a motor in which the armature is rotated only in a single direction, the electrically neutral point is changed from one position to another depending on the magnitude of a load when the motor is applied for another purpose with the load being changed from one level to another (i.e. the current passing through the armature coil being changed from one level to another). Therefore, the motor must be modified to change the position of the brush for each purpose or application.
Consideration has also been given to using an interpole, or the like, provided to reduce the adverse affect of the armature reaction. It is difficult to secure sufficient space to receive the interpole in the flat yoke type DC motor which is constructed for reducing the size thereof.
Although the flat yoke type DC motor has such problems, it can still be reduced in size and improved in power if the armature coil can be energized by an increased current, since high-performance permanent magnets capable of being used as the magnetic stator poles 12 are available today. However, it is still important to suppress the resulting armature reaction effectively, if an increased current flows through the armature.